Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling.
Further improvements in fuel economy can be achieved in engines configured to vary the effective displacement of the engine by skipping the delivery of fuel to certain cylinders in an indexed cylinder firing pattern, also referred to as a “skip-fire” pattern. One example of a skip-fire engine is shown by Tripathi et al. in U.S. Pat. No. 8,651,091. Therein, an engine fuel controller may continuously rotate which particular cylinders are fueled, which cylinders are skipped, and how many cylinders events the pattern is continued for. By skipping fuel delivery to selected cylinders, the active cylinders can be operated near their optimum efficiency, increasing the overall operating efficiency of the engine. By varying the identity and number of cylinders skipped, a large range of engine displacement options may be possible.
However the inventors herein have identified a potential issue with such engine systems. Specifically, particulate matter (e.g., soot) emissions may be degraded in such engine systems, particularly during cylinder reactivation following skip-firing (or selective cylinder deactivation in a VDE engine). As such, particulate matter (PM) emissions from spark ignited engines tend to increase when the combustion chamber surfaces are cooler. This is because fuel that reaches the cool surface evaporates more slowly, resulting in fuel films surviving on the combustion surface even after the combustion event has occurred. The fuel-rich area above the film, and the fuel evaporating from the film after the flame has passed can lead to soot formation. In addition to cold-start conditions, combustion chamber surface cooling may be accelerated during light load operation, and cylinder deactivation. Consequently, when a cylinder that was shut off for skip-firing is reactivated, there may be a tendency for noticeably higher PM emissions.
In one example, the above issue may be at least partly addressed by a method of operating an engine comprising: deactivating a first cylinder pattern of individual cylinder valve mechanisms at a first engine soot load; and deactivating a second, different, cylinder pattern of individual cylinder valve mechanisms at a second, higher, engine soot load. In this way, the skip-firing pattern of the engine may be adjusted based on the engine's soot load to keep selected engine cylinders, or all engine cylinders warm, thereby reducing exhaust PM emissions. In addition, when the cylinders are reactivated, fueling of the reactivated cylinders may be adjusted to further reduce PM emissions due to cold cylinder piston conditions.
For example, during conditions when engine coolant temperature is lower than a threshold (or soot load is higher than a threshold), and the propensity for soot production at cold cylinder combustion chamber surfaces is high, in response to a drop in torque demand, a cylinder pattern of individual cylinder valve mechanisms may be adjusted so that the periodic firing is distributed across all engine cylinders. Specifically, the cylinder pattern may be selected based on the engine configuration and cylinder firing order so that the temperature of each engine cylinder is maintained above a threshold. As such, this reduces cylinder cooling during the period of cylinder deactivation. During a subsequent reactivation of engine cylinders, such as in response to a rise in torque demand, if the engine coolant temperature is still lower than the threshold, or the engine soot load is higher than the threshold, the reactivated cylinders may be operated with split fuel injection and retarded fuel delivery for a duration to reduce soot emissions from the reactivated cylinders. This may include fueling as multiple intake stroke injections and/or a combination of intake and compression stroke injections. At the same time, the remaining active engine cylinders may continue to be operated with single fuel injection at nominal injection timing. The number of injections per engine cycle in the split fuel injection, the amount of injection timing retard, as well as the number of engine cycles over which the split fuel injection is continued for each reactivated cylinder may be adjusted based on the cylinder pattern applied as well as the number of combustion events skipped in each reactivated cylinder during the preceding deactivation. In doing so, each cylinder's temperature may be brought above a level that generates soot emissions during the reactivation.
In this way, by adjusting the pattern of cylinder deactivation response to engine soot load and engine coolant temperature, a combustion surface temperature of cylinders may be better controlled during the cylinder deactivation. By maintaining cylinders sufficiently warm during the cylinder deactivation, the likelihood of high PM emissions from the cylinders upon subsequent reactivation is reduced. In addition, the number of times that cool cylinders have to reactivated is reduced, extending cylinder deactivation benefits. By further operating reactivated cylinders with split fuel injection for a number of combustion events during a reactivation, further improvements in PM emissions is achieved while also improving the restart combustion stability of the reactivated cylinders. By injecting the fuel as multiple intake stroke injections (or an intake stroke and an early compression stroke injection), and by retarding the start of injection timing, the momentum of the fuel spray is reduced, decreasing the likelihood of the fuel spray wetting the combustion surface. In addition, cylinder heating following the cylinder deactivation can be expedited, providing emissions benefits. Overall, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.